1. Field of the Invention
The invention relates to an optical system of a microlithographic projection exposure apparatus. In particular, the invention relates to an optical system of a microlithographic projection exposure apparatus which makes possible, for different polarization distributions, an effective compensation of an undesirable system retardation with comparatively little complexity.
2. Prior Art
Microlithography is used for producing microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. In this case, the image of a mask (=reticle) illuminated via the illumination device is projected, via the projection lens, onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
During the operation of a microlithographic projection exposure apparatus, there is a need to set defined illumination settings, i.e. intensity distributions in a pupil plane of the illumination device, in a targeted manner. Alongside the use of diffractive optical elements (so-called DOEs), the use of mirror arrangements is also known for this purpose, e.g. from WO 2005/026843 A2. Such mirror arrangements comprise a multiplicity of micromirrors that can be set independently of one another.
Various approaches are furthermore known for setting specific polarization distributions in the pupil plane and/or in the reticle in a targeted manner in the illumination device for the purpose of optimizing the imaging contrast. In particular, it is known, both in the illumination device and in the projection lens, to set a tangential polarization distribution for high-contrast imaging. “Tangential polarization” (or “TE polarization”) is understood to mean a polarization distribution for which the oscillation planes of the electric field strength vectors of the individual linearly polarized light rays are oriented approximately perpendicularly to the radius directed to the optical system axis. By contrast, “radial polarization” (or “TM polarization”) is understood to mean a polarization distribution for which the oscillation planes of the electric field strength vectors of the individual linearly polarized light rays are oriented approximately radially with respect to the optical system axis.
With regard to the prior art, reference is made by way of example to WO 2005/069081 A2, WO 2005/031467 A2, U.S. Pat. No. 6,191,880 B1, US 2007/0146676 A1, WO 2009/034109 A2, WO 2008/019936 A2, WO 2009/100862 A1, DE 10 2008 009 601 A1 and DE 10 2004 011 733 A1.
During the operation of a projection exposure apparatus, the problem occurs, inter alia, that on account of birefringence occurring in the material of the optical components such as e.g. lens elements (e.g. intrinsic birefringence in calcium fluoride), stress birefringence (owing to mounts or material stresses in the production process) or layer birefringence, an undesirable system retardation occurs which results in a reduction of the so-called IPS value (i.e. the “Intensity in Preferred State”). “Retardation” is understood to mean the difference in the optical path lengths of two orthogonal (i.e. mutually perpendicular) polarization states.
Compensation of this system retardation for the purpose of increasing the IPS value can prove to be complex in this case in so far as the retardation has a vectorial character in so far as light rays having mutually different polarization states effectively experience different retardations, the system retardation moreover having a comparatively pronounced field and pupil dependence.